1. Field of the Invention
This invention generally relates to a semiconductor laser output control circuit and in particular to such a control circuit for controlling a light output of a semiconductor laser for use in an imaging apparatus, such as a laser beam printer or a laser facsimile machine.
2. Description of the Prior Art
An imaging apparatus, such as a laser beam printer, which uses semiconductor laser as a light source for emitting a light beam carrying image information is well known. In a prior art semiconductor laser output control circuit, the light output of a semiconductor laser is detected by a light detecting circuit and a detected value is compared with a reference value by a comparator, thereby causing an up/down counter to serve as an up counter or down counter to have current flow toward the semiconductor laser in accordance with the count. However, even if the laser driver current is adjusted to be maintained at constant by detecting the light output from the semiconductor laser in this manner, the light output fluctuates due to the so-called thermal coupling, and the light output at the time of light emission from a semiconductor laser (i.e., at the rising edge of an on/off control of a semiconductor laser) rises above a predetermined light output level determined by a laser drive current value and then settles down to the predetermined level after a predetermined time constant. As a result, the light output of the semiconductor laser fluctuates, which could lead to density irregularities in a resulting image if it were used in a laser beam printer as a light source.
Under the circumstances, it has been proposed to supply correction current having a predetermined time constant to a semiconductor laser so as to suppress fluctuations in the output of a semiconductor laser. In accordance with this approach, a modulation signal is integrated to produce a correction signal which is used to correct the laser drive current to be supplied to the semiconductor laser. However, in this correction method using an integrating circuit, since the time constant of such a correction has a relatively large, correction cannot be carried out rapidly and it is not suitable for correction of a short period of time, such as a few micro-seconds or less.